Closing the Winter Gap-Year-Round Measurements of Soil CO2 Emission Sources in Arctic Tundra

Non-growing season CO2 emissions from Arctic tundra remain a major uncertainty in forecasting climate change consequences of permafrost thaw. We present the first time series of soil and microbial CO2 emissions from a graminoid tundra based on year-round in situ measurements of the radiocarbon content of soil CO2 (Delta(CO2)-C-14) and of bulk soil C (Delta C-14), microbial activity, and temperature. Combining these data with land-atmosphere CO2 exchange allows estimates of the proportion and mean age of microbial CO2 emissions year-round. We observe a seasonal shift in emission sources from fresh carbon during the growing season (August Delta(CO2)-C-14 = 74 +/- 4.7 parts per thousand, 37% +/- 3.4% microbial, mean +/- se) to increasingly older soil carbon in fall and winter (March Delta(CO2)-C-14 = 22 +/- 1.3 parts per thousand, 47% +/- 8% microbial). Thus, rising soil temperatures and emissions during fall and winter are depleting aged soil carbon pools in the active layer and thawing permafrost and further accelerating climate change.